Fetal dopplers are hand held devices that transmits the sounds of the baby's heart rate and allows you to hear your baby's heart rate through a speaker or headphones. It, also helps you monitor and track your baby's progress anytime you feel like it so you can feel safe and reassured of your baby's progress. However, this device should never replace actually going to each and every one of your doctor's appointment. Best used on babies who are at least 30 weeks old; however, it can be used on babies that are as young as 12 weeks old.